1. The Causes of Weather Meteorology is the study of The Causes of Weather Weather and Climate Weather is the current state of the Climate describes the average weather over a

2. A Question of Balance In meteorology, a crucial question is how solar radiation is distributed around the planet. The Causes of Weather The Sun feels hotter in the tropics because its Because the Sun’s rays are more spread out when they strike Earth at a low angle, the same amount of energy is spread over a larger area.

3. Air Masses An air mass is a large body of air that takes on the characteristics of the area over which it forms. The Causes of Weather

4. Air Masses Classifying Air Masses The Causes of Weather –Air masses are classified according to their source regions. –The main types of air masses are: warm and dry - warm and humid - cold and dry – cold and humid - very cold and dry -

5. Air Masses Source Regions The Causes of Weather –All five main types of air masses can be found in North America because of the continent’s proximity to the source regions associated with each air mass. Maritime polar air forms over the Continental polar air forms over the The origins of maritime tropical air

6. Air Masses Source Regions The Causes of Weather –All five main types of air masses can be found in The desert Southwest and Mexico are the source regions of continental tropical air. –The stability of air is an Arctic air develops over latitudes above 60°N in the ice- and snow-covered regions of Siberia and the Arctic Basin.

7. Weather Systems The Coriolis effect, which is a result of Earth’s rotation, causes moving particles such as Weather Systems

8. Global Wind Systems There are three basic zones, or wind systems, in each hemisphere. Weather Systems The trade winds, the first major wind zone, flows at

9. Global Wind Systems Weather Systems

10. Global Wind Systems Other Wind Zones Weather Systems –The prevailing westerlies, the second major wind zone, flows between –The prevailing westerlies are responsible for much of the movement of weather across the United States and Canada. –The polar easterlies, the third major wind zone, –In both hemispheres, the polar easterlies are characterized by cold air.

11. Jet Streams Jet streams are narrow bands of high-altitude, westerly winds that flow at speeds up to 185 km/h at elevations of 10.7 km to 12.2 km. Weather Systems –The polar jet stream separates the –The subtropical jet stream is

12. Fronts In the middle latitudes, air masses with different characteristics sometimes collide, forming a front. Weather Systems A front is the narrow region separating The interaction between the colliding air masses can bring dramatic changes in weather. There are four main types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.

13. Fronts Cold Fronts Weather Systems –In a cold front, cold, dense air displaces

14. Fronts Warm Fronts Weather Systems –In a warm front, advancing –The warm air develops a gradual frontal slope rather than a steep boundary. –A warm front is

15. Fronts Stationary Fronts Weather Systems –A stationary front is the result of –Stationary fronts seldom have

16. Fronts Occluded Fronts Weather Systems –An occluded front is the result of a –Precipitation is common on both

17. Pressure Systems High-Pressure Systems Weather Systems –In a high-pressure system, –The Coriolis effect causes the overall circulation around a high-pressure center to move in a clockwise direction in the northern hemisphere. –High-pressure systems rotate in a

18. Pressure Systems Low-Pressure Systems Weather Systems –In a low-pressure systems, air rises, causing an –In contrast to air in a high- pressure system, air in a low- pressure system in the northern hemisphere moves in a counterclockwise direction. –This movement is

19. Gathering Weather Data Meteorologists measure the Gathering Weather Data Two of the most important factors in weather forecasting are

20. Surface Data A thermometer is a device used to Gathering Weather Data A barometer is a device used to – An anemometer is used to – A hygrometer measures – A ceilometer measures the

21. Upper-Level Data To make accurate forecasts, meteorologists must gather atmospheric data at heights of up to 30 000 m. Gathering Weather Data A radiosonde, a The sensors on a radiosonde The radiosonde is also tracked to determine wind speed and direction at various altitudes.

22. Weather Radar Doppler Radar Gathering Weather Data –The Doppler effect is the change in –Meteorologists use Doppler radar, which is –Because the motion of the moving raindrops is caused by wind, Doppler radar provides a good estimation of the wind speeds associated with precipitation areas, including those that are experiencing severe weather such as thunderstorms and tornados.

23. Weather Analysis A station model is a record of Weather Analysis Meteorological symbols are used to represent weather data in a station model.

24. Surface Analysis To plot data nationwide or globally, meteorologists use isopleths. Weather Analysis Isopleths are lines that –Lines of equal pressure are called –Lines of equal temperature are called

25. Surface Analysis Isobars also indicate the Weather Analysis

26. Short-Term Forecasts Digital Forecasts Weather Analysis –A digital forecast is a forecast